Scriptwriting isn't easy. Some people spend months trying to perfect a specific scene, trying to write that perfect line that sticks in the mind of those who hear it. Then these guys come along and do it off the top of their heads. -- Marc Russel

THE SILENCE OF THE LAMBS

Anthony Hopkins' performance as the insane doctor Hannibal Lecter is by far his most notorious. He got an Oscar for the part in spite of only having 24 minutes of screen time -- just because he's that creepy. The above clip shows probably his most famous line from the film. While this line was in fact in the script, his hissing after was not, and surprised everyone on set particularly Jodie Foster. That disturbed look on her face there? That wasn't acting - she was genuinely creeped out.

TAXI DRIVEREvery great movie has one line or moment that will forever be associated with that film. In Taxi Driver, that line is "You talkin' to me?" It actually became referenced in pop culture so often it was declared by the American Film Institute to be the tenth greatest movie quote of all time. It wasn't even in the script. De Niro was just supposed to spend a moment looking menacingly at his reflection in the mirror. Instead he spent a full minute threatening it out loud.

THE SHINING

Like the above Taxi Driver quote, this line will always be linked to its movie in the cultural unconscious. Like the above quote, it's on the AFI's best quotes list. And like the above quote, it was totally improvised. All Jack Nicholson was supposed to do was break the door down, but he figured that wasn't quite pants-crappingly insane enough, so he screamed the then-famous intro to The Tonight Show. It didn't even make sense in context, since his character's name was Jack Torrance. But holy hell was it effective.

FULL METAL JACKET

R. Lee Ermey was actually a real drill sergeant originally hired to be a technical advisor. When he asked to be allowed to audition for the part, Stanley Kubrick said he didn't seem mean enough for it. Ermey responded by sending him a 15-minute video clip of him standing in front of a camera screaming an endless stream of insults at some Marines while being pelted with tennis balls. It's not entirely clear why he was being pelted with tennis balls, but it was probably to prove a point about how completely balls-to-the-wall insane he was. Almost the entire scene where the sergeant was introduced was made up on the spot. Partway through, Kubrick had to stop the filming to ask Ermey what the hell a "reach-around" was.

It's worth noting that that isn't the only part of the movie that was improvised. About half of all the lines the Drill Sergeant has were completely made up on the spot. Kubrick estimates that about 150 pages of the script were just off the top of Ermey's head.